(a) Field of the Invention
The present invention relates to a method for preparing cerium oxide (CeO2) nanoparticles, and more particularly to a method for preparing single crystalline cerium oxide nanopowders having a particle size of not less than about 30 nm that were difficult to synthesize in conventional hydrothermal reactions, as well as having a uniform particle shape and particle size distribution and being capable of being produced at a low temperature.
(b) Description of the Related Art
Cerium oxide (CeO2) powders are multifunctional powders that are applied to abrasives, catalysts, fluorescent materials, etc. As the semiconductor industry has developed, cerium oxide powders have been spotlighted as a main component of slurry that is used in a CMP (Chemical Mechanical Polishing) process of the manufacture of semiconductors.
In many fields where cerium oxide (CeO2) powders are applied, the synthesis of the cerium oxide (CeO2) powders (whose particles are fine, uniform, and spherical) is urgently required, but there is no special method for preparing the cerium oxide powders except for a high-temperature, high-phase synthesis where particle size and dispersion are difficult to control. Further, there has been no research regarding the synthesis of single crystalline cerium oxide powders of not less than 30 nm by using various solution methods such as the coprecipitation method, hydrothermal synthesis, the emulsion method, etc. The solution methods easily control granularity and shape. Thus there are several difficulties in utilizing cerium oxide (CeO2) powders.
It has been recognized that it is not possible to achieve excellent characteristics of ceramics in conventional methods where powders are obtained by the comminution of minerals. Liquid phase powder synthesis such as in the coprecipitation method, the sol-gel method, hydrothermal synthesis, etc. has been studied to develop novel characteristics of ceramics and obtain ceramic products having a high added value, by supplementing the drawback of the conventional procedures.
In particular, research of hydrothermal synthesis has been increased, because the hydrothermal synthesis not only has particle size and shape control characteristics of liquid phase powder synthesis, but it also controls granularity and shape of single crystalline particles with growing the single crystalline particles in the state of solution at a much lower temperature than that of solid phase reaction.
On the other hand, as liquid phase powder synthesis such as hydrothermal synthesis is conducted in a build-up manner where a small nucleus is grown into a large particle, difficulties exist in synthesizing large particles having high crystallinity even though fine particles can be comparatively easily synthesized. So as to overcome this problem, several attempts have been conducted. For instance, after the size of initial starting particles is controlled using a seed, the particles are merely grown into crystals; or high-temperature, high-pressure reaction in a supercritical state above the critical point of water is applied; or an acid and/or a base having a high concentration are used to increase solubility. But the attempts still have problems. Particularly, in the case of supercritical fluid method using supercritical water, expensive equipment capable of being applied to a high temperature reaction is required, turnover of expensive parts is short, and the control of reaction condition is difficult. Thus the industrial application of the attempts is still far off in spite of continuous studies thereof.
The powder synthesis process via a solution phase generally comprises two steps: nucleation and growth of crystals. In order to regulate the size of particles, both steps must be well controlled, and particularly in the step of nucleation, the greater the number of nucleuses, the smaller the size of particles becomes; and in the growth of crystals, secondary nucleation occurs in the case that supersaturation is high or the growth into large particles requires a lower energy barrier than that of the nucleation. Accordingly, particles having uniform and large crystals are difficult to generate. In general, to obtain large and uniform particles, the supersaturation of reaction solution must be suitably controlled. This supersaturation can be controlled mostly by the concentration of solute and the solubility of solution. Therefore, so as to synthesize desired ceramic powders, it is very important to choose a suitable solvent, and the solute's concentration, temperature, and additives for regulating solubility and regulating the shape of particles.
Matijevic et al. disclosed power synthesis of CeO2 via solution phase in which hexagonal plate and spherical cerium oxide particles were prepared by sealing Ce(SO4)2.4H2O, (NH4)4Ce(SO4)4.2H2O, (NH4)2Ce(NO3)6, etc. as starting materials in a sealable Pyrex tube, heating them at a constant temperature to thereby precipitate cerium hydroxide and then calcining the cerium hydroxide at a temperature of about 600° C. (Wan Peter Hsu, Lena Roannquist, Egon Matijevic, Preparation and Properties of Monodispersed Colloidal Particles of Lanthamide Compounds. 2. Cerium(IV), Langmuir, 4, 31-37 (1988)).
Also, E. Tani, et al. synthesized cerium oxide powders of about 100 μm or larger by precipitating hydroxide using cerium nitrate and NH4OH as starting materials, and hydrothermally synthesizing the hydroxide together with various additives at a high temperature of about 500 to 600° C. (E Tani, M. Yoshimura, S. Somiya, Crystallization and crystal growth of CeO2 under hydrothermal conditions, J. Mater. Sci. Letters, 1, 461-462, (1982)).
Also, Takuya Tsuzuki, et al. synthesized uniform nanosized cerium oxide by the mechanochemical process and calcination process of using cerium chloride (CeCl3) and NaOH as starting materials. In the first comminution process, cerium hydroxide was synthesized via the mechanochemical reaction by comminuting cerium chloride, NaOH, and NaCl using a steel ball, and the cerium hydroxide was calcined at a temperature above 500° C., whereby a spherical nanosized cerium oxide was synthesized.
However, the synthesis of cerium oxide particles by such a mechanochemical process contains a large quantity of sodium, which is a fatal contaminant in semiconductor processes, and thus the addition of a separate washing steps is inevitable. Additionally, because of agglomeration and crystallization due to the calcination process, a large amount of energy is consumed during the comminution into nanosized particles. Therefore, with respect to its application to industrial and CMP process, problems still exist to be solved (Takuya Tsuzuki, Paul G. McCormick, Synthesis of Ultrafine Ceria Powders by Mechanochemical Processing, J. Am. Ceram. Soc., 84(7), 1453-58, (2001)).